This application is a continuation-in-part of application Ser. No. 10/701,274, filed Nov. 4, 2003, now U.S. Pat. No. 6,825,450.
Various methods are known for bonding separately formed portions of a piston in order to yield a piston structure. One such process is friction welding in which one portion of the piston is rotated at high speed while pressed against the other portion, with the resulting frictional energy generating sufficient heat to bond the portions together. Other techniques include resistance welding, induction welding, and the like in which, after the portions are brought into contact with one another, an energy flux is introduced across their joining surfaces which causes them to be heated sufficiently to join the surfaces to one another.
U.S. Pat. No. 5,150,517 is an example of friction welding, whereas U.S. Pat. No. 6,291,806 is an example of typical induction heating wherein the coils are presented to the sides of the contacting joining surfaces to induce energy and thus heat at the interface. Such side presentation of the induction coils has a tendency to heat the regions of the joining surfaces near the edges of the material adjacent the induction coils at a faster rate than those regions further from the coils, thus producing a variation in the heat flow and heat affected zone in the area of the material adjacent the interface. In a demanding, highly loaded application such as pistons for diesel engines, it would be desirable to provide a weld joint that is uniform in its heat affected zone across the interface so as to minimize any variation in strength and integrity of the material.
U.S. Pat. No. 6,155,157 discloses a piston having first and second portions which are joined across two radially spaced sets of joining surfaces by means of friction welding. It will be appreciated that such an architecture would present a challenge to joining the portions by induction welding, since access to the regions where the joining surfaces are located is limited and, in the case of the internal cooling gallery, inaccessible to the positioning of any induction coil next to the mated joining surfaces. Based on the known existing technology in the field of pistons, a suitable technique for induction welding such complex architectures of pistons as those shown in the aforementioned '642 patent is not known to be in existence, and certainly is not known to be used due to the practical difficulties in adapting such induction heating technology to complex piston designs with multiple radially spaced joining surfaces.
Outside of the field of heavy-duty pistons, induction heating is used to join simple structures, such as butt-welding metal tubes that carry petroleum products. U.S. Pat. No. 6,637,642 discloses such a process. Such tubing is a simple, single walled cylindrical structure having flat, planer end faces. To join one end face to another, an induction coil is introduced between the end faces, and the end faces are heated to an elevated temperature, after which the coil is withdrawn and the end faces brought into engagement with one another to achieve a weld joint. Preferably, once the surfaces are brought into contact, they are twisted a small amount (a few degrees) to attain more intimate union of the weld surfaces. Surprisingly, the inventors have discovered that the induction welding technique heretofore limited to joining simple single walled cylindrical petroleum piping can be improved to be successfully employed to join complex piston structures in a manner to attain a strong, high integrity joint with a uniform but minimal heat affected zone across the interface of the joining surfaces.